This invention relates generally to devices for digital electronic scanning of images and more specifically to detection of dust and scratches and other surface defects.
Electronic image scanners convert an optical image into an electronic form suitable for storage, transmission or printing. In a typical image scanner, light from an image is focused onto linear arrays of photosensors for scanning one line at a time. A two dimensional image is scanned by providing relative movement between the linear sensor arrays and the original image. For gray-scale scanning there may be only a single linear array of photosensors. In general, a color scanner measures the intensity of at least three relatively narrow bands of wavelengths of visible light, for example, bands of red, green and blue.
For image scanners, the digitized image may be degraded by the presence of artifacts on the surface of the object being scanned, such as dust and fingerprints, or defects in the surface of the object being scanned, such as scratches, folds, or textured surfaces. Multiple methods have been disclosed for detecting defects on transparent media. See, for example, U.S. Pat. No. 5,266,805, U.S. Pat. No. 5,969,372, and EP 0 950 316 A1. Some of the methods in the referenced patent documents utilize the fact that the dyes in transparent color film are essentially transparent to infrared light, whereas dust and scratches are relatively opaque. Other disclosed methods utilize dark field imaging, in which the light reaching the photosensors is reflected or diffracted by defects instead of the film.
Scanners for opaque media are configured differently than scanners for transmissive media, and different detection methods are needed. There is a need for automatically distinguishing surface artifacts and defects on reflective media.
Multiple scans of the same object are obtained, where for any given line on the object to be scanned, the angle of the illumination is different for each scan. The different scans are obtained from different rows of photosensors that are separated. Because the angles of illumination are different, the resulting shadows in each scan are different. The multiple scans may be combined into a single composite color image. In a composite image, a dust particle may generate a series of overlapping shadows, where each shadow is a different color. Searching the composite image for the unique pattern of colors may identify artifacts or defects. Alternatively, the data for one scanned image may be compared to the data for another scanned image, and any differences may be due to shadows, which may indicate defects.